Chapter 19

1. Chapter 19 In the Beginning

2. Introduction • By itself, the expansion of the Universe does not prove that there was a big bang; indeed, one could postulate that the Universe had no beginning in time and will have no end. • The fatal blow to this “steady-state theory” was the discovery of a faint radio glow that pervades all of space and was produced when the Universe was very young. • The existence of a nearly uniform amount of helium and deuterium (heavy hydrogen) throughout the Universe provides additional evidence for a very hot, dense phase in its early history.

3. Introduction • But how did the Universe achieve a uniform temperature, and why is space so nearly flat? • These troubling issues led to a magnificent but still unproven hypothesis: The early Universe apparently went through a stage of phenomenally rapid expansion, doubling its size many times in just a tiny fraction of a second. • Such a process may have created the Universe from essentially nothing! • And if it happened once, it might happen again; perhaps there exist multiple “universes” physically disconnected from ours. • We emphasize from the start that these last few ideas are quite speculative, much more so than the other material presented in this book. • Nevertheless, based on what we now know, they are physically reasonable possibilities.

4. 19.1 The Steady-State Theory • In the previous chapter, we considered the big-bang theory of the Universe, which is motivated in part by the observed recession of galaxies away from our own Milky Way Galaxy (see figure). • We saw that the matter density of the Universe dictates the history, as well as the future, of its expansion rate. • Moreover, there is growing evidence for a cosmic “antigravity” effect over large scales that accelerates the expansion of the Universe, causing it to grow larger and less dense at an ever-increasing rate.

5. 19.1 The Steady-State Theory • Although the expansion of the Universe suggests that there was a definite beginning of time when matter was in a very compressed and hot state, this is not the only logically consistent possibility. • Here we consider a reasonable alternative that turne out to be incorrect, but that nevertheless served science well because it forced cosmologists to question critically their assumptions and conclusions.

6. 19.1 The Steady-State Theory • In 1948, Fred Hoyle of Cambridge University (see figure) and two of his associates (Hermann Bondi and Thomas Gold) proposed the steady-state theory as an alternative to the hot big bang. • It is based on a modification of the cosmological principle called the perfect cosmological principle: The Universe is homogeneous and isotropic on large scales, and its average properties never change with time. • Therefore, there was no well-defined beginning (the Universe is infinitely old), and there will be no end; the Universe is simply expanding throughout all time. • To keep the average density constant as the Universe expands, new matter must be created continuously, and new galaxies form out of this material.

7. 19.1 The Steady-State Theory • We might argue against the steady-state theory on the grounds that it requires something to be produced from nothing, which is thought to be impossible over long timescales. • That is, we believe in the law of “conservation of energy”—the combination of mass plus energy is neither created nor destroyed. • But this objection turns out to be observationally weak. Only one hydrogen atom must be created per cubic meter, per billion years, to satisfy the requirements of the steady-state theory. • This rate is well below the current detection limit of laboratory experiments. • In other words, we have not yet verified the law of conservation of energy to this degree of accuracy. • Moreover, the original big-bang theory postulates that the Universe was created out of nothing in a single instant (t = 0), so it too appears to violate conservation of energy (although we will later see that no significant violation might occur).

8. 19.1 The Steady-State Theory • Despite its aesthetic appeal to some astronomers, the steady-state theory gradually lost support as observational evidence showed that the Universe has a finite age (about 14 billion years, not much greater than the age of the oldest globular clusters) and that its properties change with time. • One of the first arguments for evolution of the Universe was that radio galaxies seem to be more numerous and luminous at large distances. • An even more compelling case was made with quasars (see Chapter 17): They are clearly denizens of the distant past; all have high redshifts, so we see them at large lookback times. • There are no nearby quasars, seen as they are in the present-day Universe, but only lower-luminosity active galaxies. • In recent years, Hubble Space Telescope images of distant galaxies have shown that they differ from nearby galaxies, providing additional evidence for the evolution of the Universe.

9. 19.2 The Cosmic Microwave Radiation • The fatal blow to the steady-state theory, however, was the discovery of the cosmic microwave radiation—a faint afterglow from the Universe’s hot past. • The steady-state theory has no explanation for it, but big-bang models require it, as we will now discuss.

10. 19.2a A Faint Hiss from All Directions • In 1964 through 1965, two young researchers (Arno A. Penzias and Robert W. Wilson) were observing with a 20-foot horn-shaped radio antenna owned by Bell Labs near Holmdel, New Jersey (see figure). • The antenna had previously been used to detect radio signals from communications satellites, but Penzias and Wilson were allowed to use it for astronomical observations. • They initially wanted to accurately measure the radio brightness of Cas A, a prominent supernova remnant that is used to calibrate radio data. • Then they wanted to map the 21-cm radio emission in the Milky Way Galaxy.

11. 19.2a A Faint Hiss from All Directions • For the most accurate measurements, they needed to make their microwave antenna as sensitive as possible. • After removing all possible sources of noise, they found a very faint but persistent hiss that seemed to defy explanation. • It was independent of location in the sky, and also independent of the time of day. • If produced by a “black body” (Chapter 2), it corresponded to a temperature of about 3 K, which is just 3°C above absolute zero and is equal to –270°C. • They were not aware that it might be the afterglow of the big bang! • Meanwhile, at nearby Princeton University in New Jersey, a team led by Robert Dicke was building a radio telescope with which to detect this afterglow. • They had predicted that it should exist if the Universe began in a very hot and compressed state.

12. 19.2a A Faint Hiss from All Directions • Actually, this prediction had already been made in the 1940s, by the Russian astrophysicist George Gamow (who was then working in the United States), but the Princeton group was unaware of it. • Gamow and his students, Robert Herman and especially Ralph Alpher, had based their conclusion on the idea that all of the chemical elements were created shortly after the big bang. • Although incorrect in detail, this is correct in spirit; only the lightest elements were produced early in the Universe, as we shall see later. • The Universe must have been very hot to do this, but expansion would have cooled it, so the radiation should now be that of a cold black body.

13. 19.2a A Faint Hiss from All Directions • Another astronomer alerted the Princeton and Bell Labs teams to each other, and that’s how Penzias and Wilson found out what they had discovered. • The Princeton group made a measurement at another wavelength, and it agreed with the prediction of a black-body spectrum. • Other, subsequent measurements were also consistent with black-body radiation from a gas at T  3 K, a very low temperature indeed. • In 1990, astronomers announced results from NASA’s Cosmic Background Explorer (COBE, pronounced “koh´bee”; see figure, top): The spectrum was that of a perfect, cold black body (see figure, bottom).

14. 19.2b Origin of the Microwave Radiation • From where, exactly, does a photon of the cosmic microwave background radiation come? • The early Universe was very hot and ionized: There were no bound atoms, but rather free electrons and atomic nuclei. • Because photons easily scatter (bounce, or reflect) off of free electrons, they cannot travel in a straight line from one object to another, and instead jump randomly from one direction to another. • So instead of being transparent, the Universe looked opaque, like thick fog. • Even if there were discrete objects at this time, one would not be able to see them. • The Universe was filled with photons, however, since it was hot—and they simply scattered around randomly in a sea of hot particles.

15. 19.2b Origin of the Microwave Radiation • As the Universe expanded, it cooled. Eventually, about 400,000 years after the big bang, it reached a temperature of about 3000 K, and electrons were able to combine with protons to form neutral hydrogen atoms. • The process is called “recombination”—even though in this case, electrons were combining with protons, forming neutral atoms for the first time in the history of the Universe. • Unlike free electrons, electrons bound in atoms are able to interact with photons of only certain specific energies, absorbing them (Chapter 2). • They are not effective at scattering photons at other energies. • Thus, at this time most photons stopped bouncing around, and became free to travel unhindered. • The Universe therefore became transparent to almost all electromagnetic waves. • Astronomers say that matter and radiation “decoupled” from each other at t  400,000 years.

16. 19.2b Origin of the Microwave Radiation • As space expanded, the wavelengths of photons increased, just like that of a wavy line drawn on an expanding balloon, so they lost energy. • They maintained the spectrum of a black body, but of lower and lower temperature (see figure). • Typical photons went from being optical /infrared to radio (microwave). • The cosmic microwave radiation now corresponds to such a low temperature, about 3 K, because the Universe has expanded a great deal over its 14-billion-year life. • Since it started off as radiation filling the entire Universe, it is easy to understand why it now comes equally from every direction (that is, it appears isotropic).

17. 19.2b Origin of the Microwave Radiation • In essence, the cosmic background photons come from an opaque “wall” at redshift about 1000 (the ratio of 3000 K to 3 K). • We cannot see electromagnetic radiation from beyond this wall because the Universe was opaque. • This marks the “boundary” of the observable Universe: We cannot see electromagnetic radiation from times prior to about 400,000 years after the big bang. • In principle, we could see neutrinos from t  400,000 years, since the Universe was not opaque to them. • But unfortunately, neutrinos are very difficult to detect.

18. 19.2b Origin of the Microwave Radiation • The cosmic microwave radiation is left over from when the Universe was very hot, dense, and opaque. • Being almost perfectly isotropic, it provides the best-known evidence in support of the cosmological principle that the Universe is homogeneous and isotropic. • The steady-state theory has no reasonable explanation for its presence. • Penzias and Wilson shared the 1978 Nobel Prize in Physics for their important discovery: They had been very careful, and had not ignored the faint (almost nonexistent) hiss. • Gamow could not receive the Nobel Prize, as he had already died, but it seems somewhat unfair that Alpher was omitted. (It can be shared among a maximum of three people.)

19. 19.3 Deviations from Isotropy • The cosmic microwave background shows a slight deviation from isotropy of about two parts per thousand. • It looks slightly hotter in one direction of the sky, and slightly cooler in the opposite direction. • Such an “anisotropy” is caused by a combination of motions: the Sun’s motion around the center of the Milky Way Galaxy, the Milky Way’s motion within the Local Group, and, most important, the Local Group’s motion relative to the “Hubble flow” (that is, relative to the smoothly expanding Universe).

20. 19.3 Deviations from Isotropy • These gravitational perturbations are all known. • The sum of the motions produces a net motion in a particular direction of space, and the slight temperature anisotropy is attributed to the Doppler effect. • The radiation coming from the direction of motion is slightly blueshifted (“hotter”), while that coming from the opposite direction is slightly redshifted (“cooler”). • Maps made with COBE very clearly show this anisotropy (see figure).

21. 19.3a Ripples in the Cosmic Microwave Background • A major puzzle for many years was the apparent absence of small variations in the background radiation corresponding to the size scales of clusters and superclusters of galaxies. • If these structures formed by the gravitational contraction of matter, there should have been slight ripples in the density of matter early in the Universe. • Photons escaping from higher-density clumps would have a slightly different redshift than those coming from regions of average or below-average density. • There are several relevant processes, but perhaps the easiest to understand is that photons lose more energy when they come out of stronger gravitational fields. • The different redshifts would translate into slight variations in the associated temperature of the radiation.

22. 19.3a Ripples in the Cosmic Microwave Background • No such ripples were found until the early 1990s, despite extensive searches. • Some specific models of cluster formation were consequently eliminated from further consideration, because they predicted variations larger than the observed upper limits. • In 1992, a breakthrough was made with the COBE satellite: Tiny temperature variations (about one part per hundred thousand, or 30 microkelvins) were finally found, at least in a statistical sense. (Ripples were present, but no great significance could be given to any particular one.) • After a few more years, the data became quite convincing (see figure). • The angular resolution of COBE was low, so even the smallest detected ripples correspond to very large superclusters of galaxies.

23. 19.3a Ripples in the Cosmic Microwave Background • Subsequently, several balloon-based experiments found similar variations, but on smaller angular scales (see figure), corresponding to large clusters of galaxies. • These are the “seeds” from which large-scale structure grew in the Universe. • They are the imprints of minuscule ripples in the distribution of matter established shortly after the big bang. • One of the missions was BOOMERANG: Balloon Observations of Millimetric Extragalactic Radiation and Geophysics. • The results showed that the Universe is flat (see figure), as we will now discuss.

24. 19.3b The Overall Geometry of the Universe • By 2001, several missions had shown that the angular size of temperature variations in the cosmic background radiation is typically about one degree (1°; see figure). • This statement means that the typical angular size of a ripple is around 1°, and it tells us something about the overall geometry of the Universe. • In fact, the data are consistent with the presence of considerable amounts of dark matter and other energy in the Universe—enough to make the Universe spatially flat! (Looking at the data showed that the Universe is flat, but analysis of the sizes of the fluctuations gives us much more detail and some explanation as to why and how.)

25. 19.3b The Overall Geometry of the Universe • A spatially flat geometry means that , the ratio of average total density (of matter, normal energy, and other energy) to the critical density, is 1 (see Chapter 18). • This “other energy” may well be the “vacuum energy” or “dark energy” associated with the cosmological constant, whose nonzero value was first suggested by observations of distant supernovae (see Chapter 18). • Galaxies and clusters of galaxies seem to have formed at peaks in the overall distribution of dark matter. • Results announced in 2003 show the fluctuations at still better resolution (see figure). • From the results, scientists infer an amount of dark energy matching that determined from supernova observations that led to the accelerating-universe conclusion.

26. 19.3b The Overall Geometry of the Universe • Other observations show that the cosmic microwave background is polarized—that is, radiation coming from a given region of the sky has more of its electric field oscillating in one direction than in the perpendicular direction. • Such polarization is typically produced when light scatters off of a surface. • For example, light reflected from water or pavement is polarized, and polarizing sunglasses reduce the glare by blocking light having the dominant direction of oscillation. • The observed polarization tells us that the cosmic background radiation is indeed coming from a surface of last scattering, a moment before recombination occurred and the Universe became transparent.

27. 19.3c The Wilkinson MicrowaveAnisotropy Probe (WMAP) • A NASA spacecraft now aloft, the Wilkinson Microwave Anisotropy Probe (WMAP), is covering the whole sky at 30 times the resolution of COBE. • It was launched in June 2001 into an orbit a million miles from us, on the side of the Earth opposite to the direction to the Sun. • The results of WMAP’s first year of data were released in February 2003. • The probe is named after David Wilkinson, an important member of the COBE and WMAP teams and a member of the original Princeton team that was among the first to understand the cosmic microwave background.

28. 19.3c The Wilkinson MicrowaveAnisotropy Probe (WMAP) • WMAP’s map of the entire sky (see figure) is spectacularly detailed. • It shows temperature variations measured in microkelvins—millionths of degrees. • COBE also had mapped the whole sky, but at lower resolution, and though several recent experiments had mapped at high resolution, they had covered only tiny portions of the sky. • Thus the results from WMAP, taken at five wavelengths between 2.3 mm and 13 mm, represented a major advance. • Comparing the WMAP maps at the different wavelengths allowed the effects of the Milky Way to be subtracted. • The results for a variety of cosmological parameters are so precise that they have greatly helped transform cosmology into a precision science.

29. 19.3c The Wilkinson MicrowaveAnisotropy Probe (WMAP) • From studying various features (especially the first peak) in the graph showing the distribution of the sizes of the variations, and making use of additional information from other astronomical studies, we now know the age of the Universe. • It is 13.7 billion years, with a formal statistical uncertainty of only 0.2 billion years. (However, various systematic effects tend to increase the uncertainty, perhaps to about 0.5 or even 0.7 billion years.) • This value fits with our current understanding of the main-sequence turnoff in globular clusters of 12 billion years and the ages of the oldest white dwarfs in clusters of 12.7 billion years, both with uncertainties of about 1 billion years.

30. 19.3c The Wilkinson MicrowaveAnisotropy Probe (WMAP) • WMAP’s data, in conjuction with other studies, show that Hubble’s constant is 71 km/sec/Mpc, with an uncertainty of only 6 per cent. • This value is in close agreement with the Hubble Key Project’s value of 72 km /sec/Mpc. • The long battle over the value of Hubble’s constant seems finally to be over. • The data, when linked with observations from the Hubble Key Project, the observations of distant supernovae, or maps of the distribution of galaxies in the sky, show that the Universe is flat. • The measurements are uncertain to a 66 per cent probability by only 2 per cent, and to a 95 per cent probability by only 5 per cent. • The total amount of matter and energy in the Universe, , is thus 1 to a high precision.

31. 19.3c The Wilkinson MicrowaveAnisotropy Probe (WMAP) • WMAP’s coverage allows the distribution of the sizes of temperature variations to be measured much more accurately than had previously been done. • The distribution of its peaks reveals other cosmological parameters. • In particular, the density of normal types of matter—the kinds of particles that you are made of (technically, “baryons”)—comprises only 4.4 per cent of the total. • And of that same total, only about 1 per cent is luminous matter, the type that we can detect. • Hence, the stuff of which we are made is just a small minority of all that exists; we are like the debris of the Universe. • The rest of the 4.4 per cent contributes to the dark matter that scientists have been detecting for some decades by its gravitational effects in the halos of galaxies and in clusters of galaxies. • MACHOs are an example of this type of dark matter, but recent results suggest that much of it actually consists of tenuous, million-degree gas in the halos of galaxies.

32. 19.3c The Wilkinson MicrowaveAnisotropy Probe (WMAP) • WMAP shows, further, that another 23 per cent is other types of “cold dark matter.” • In this context, “cold” means that the matter is moving slowly, not close to the speed of light. • We do not yet know what this matter consists of, but the “best bet” is exotic particles left over from the big bang. • We can tell that most of the dark matter cannot be “hot” dark (that is, moving at very high speeds) because if a substantial amount of hot dark matter had been present in the early Universe, star formation would have been suppressed at that time. • The WMAP results, in contrast, imply that the first stars formed only about 200 million years after the big bang.

33. 19.3c The Wilkinson MicrowaveAnisotropy Probe (WMAP) • Thus, normal matter and cold dark matter contribute about 0.27 to the density of the Universe, . • Since the total density is 1.0, that leaves another 73 per cent as “dark energy” (see figure), which we discussed in Chapter 18. • We have even less understanding of dark energy than we do of cold dark matter. • We appear to live in a Universe in which we can relate to only 4.4 per cent of its constituents, if even that! • As John Bahcall of the Institute for Advanced Study summarized, “We are in an implausibly crazy Universe, but one whose characteristics we know.”

34. 19.3c The Wilkinson MicrowaveAnisotropy Probe (WMAP) • The WMAP findings represent strong endorsement of the big-bang model of the Universe. • They also endorse some versions of inflationary cosmology (“inflation” will be discussed later in this chapter), though other specific inflationary scenarios are no ruled out. • Also, the “quintessence” model that had rivaled Einstein’s cosmological constant as a contender for explaining the dark energy now seems somewhat less likely than it had previously. • Spectacular as these results are, they will improve still further as the remaining three years of data are collected and analyzed. (The announcement of findings from Years 2 and 3 is anticipated in 2006.) • Yet another improvement should result from the European Space Agency’s Planck spacecraft, currently scheduled for launch in 2007.

35. 19.4 The Early Universe • The evolution of the Universe can be studied nearly back to the moment of creation, “t = 0,” with an increasing amount of uncertainty as we go to progressively earlier times. • Just how close to t = 0 we can get with reasonable accuracy is debated.

36. 19.4a Going Back in Time • Almost all astronomers and physicists agree that we can go quite far back in time because the early Universe was relatively simple: It consisted of uniformly distributed particles and photons in thermal equilibrium (that is, at the same temperature), and we seem to know the few fundamental laws that governed their behavior. • Compare this with a complex problem like Earth’s weather, which is affected by factors such as local heating, ocean currents, cloud formation, continents, mountain ranges, and so on. • It is easier to predict the properties of the early Universe than it is to predict the weather! • Certainly we can reach something like t  1 sec, because conditions were similar to those in the central regions of stars, and the nuclear physics is well understood. • We also have substantial confidence in our conclusions down to t  10-12 sec, since the corresponding temperatures and energy densities have been reproduced in high-energy particle accelerators.

37. 19.4a Going Back in Time • It can also be argued that we can extrapolate all the way to about 10-35 sec, or perhaps somewhat earlier; once again, hot gases in equilibrium are relatively easy to understand. • However, we do not yet know everything down to such small timescales; our physical theories are incomplete, and no conceivable particle accelerators can achieve temperatures that are high enough to test them. • Thus, some of our current conclusions about this era are rather speculative. • Turning this problem around, we can use celestial observations to test the physical theories: The Universe is our great “accelerator in the sky.” • Although times such as 10-35 sec may seem ridiculously short, with no real “action,” it is important to understand that lots of things could have happened. • This is because the interaction timescales and distances were far smaller than they are today.

38. 19.4b A Brief History of the Early Universe • Concerning times before 10-43 sec we know essentially nothing, except that the temperature exceeded about 1032 K. • This is the Planck time: It is thought that time itself might be packaged in small units (“quantized”) at about this interval, or at least it becomes unpredictable. • Some physicists have the notion of “space–time foam,” where packets of time and space themselves flit into and out of existence. • We must develop a self-consistent quantum theory of gravity to better understand this era. • When the Universe was between 10-35 and 10-6 sec old, there was equilibrium among particles, antiparticles, and photons. • Particle–antiparticle pairs annihilated each other and produced photons. • Photons spontaneously formed particle–antiparticle pairs. • Quarks, particles that are normally bound together by “gluons” to form protons and neutrons, were plentiful and unbound during most of this era.

39. 19.4b A Brief History of the Early Universe • Quarks come in six types or “flavors”: up, down, strange, charmed, top (or truth), and bottom (or beauty). • Each flavor also comes in three “colors”: blue, green, and red. (These names are whimsical; they don’t reflect the real “character” of each quark.) • Normal matter consists of the “up” and “down” quarks, together with electrons and electron neutrinos (a type of neutrino associated with electrons). • For example, a proton is two “up” quarks (each with a charge of +⅔e, where e is the unit of electric charge) and one “down” quark (with a charge of ⅔e). • A neutron is one “up” quark and two “down” quarks (see figures). Each quark has a corresponding antiquark.

40. 19.4b A Brief History of the Early Universe • At some stage, probably early in this era of the quark-gluon-photon mixture, a slight imbalance of matter (quarks) over antimatter (antiquarks) was formed. • It amounts to about one part per billion. • It is not known which specific reactions took place, but laboratory experiments have demonstrated the existence of a matter–antimatter asymmetry for some processes (see figure). • We owe our existence to this tiny asymmetry.

41. 19.4b A Brief History of the Early Universe • The annihilation of protons, neutrons, and their antiparticles occurred at t  10-6 sec, one microsecond, when the temperature was about 1013 K, leaving a sea of photons. • The annihilation was not complete, due to the slight imbalance of matter over antimatter. • Then there was rapid conversion of protons and neutrons back and forth into each other (see figure). • However, since neutrons are slightly more massive than protons, they are harder to make, and the number of neutrons settled to only about one quarter of that of protons.

42. 19.4b A Brief History of the Early Universe • By the time the Universe was about 1 sec old, its temperature had dropped to about 1010 K (ten billion degrees), and the typical photon energies became too low to spontaneously produce electron–positron pairs. • Electrons and positrons therefore annihilated each other for the last time, producing more photons. • The remaining (excess) electrons balanced the positive charge of the protons. • Moreover, at this time, neutrons began to decay into protons, electrons, and antineutrinos; it was not possible to rapidly replenish the supply of neutrons. • This further increased the imbalance between protons and neutrons.

43. 19.4c Primordial Nucleosynthesis • The temperature had dropped to only a billion degrees by an age of about 100 sec, allowing the formation of heavy isotopes of hydrogen, two isotopes of helium, and a little bit of lithium. • Before this time, collisions between particles would sometimes produce bound states, but subsequent collisions destroyed them. • At the temperature and density of matter when t  100 sec, collisions tended to produce a surplus of bound particles; they were not immediately destroyed (see figure). • Elements heavier than lithium were not formed at this time: The temperature and density dropped too rapidly. • By an age of about ten minutes, the Universe had completed its primordial nucleosynthesis—the formation of the light elements shortly after the big bang.

44. 19.4c Primordial Nucleosynthesis • Primordial nucleosynthesis is predicted to have happened when the proton to neutron ratio was about 7:1. • For every 16 nuclear particles, 2 were neutrons, and these generally combined with 2 protons to form helium, thereby using up 4 of the 16 particles. • The other 12 particles remained as protons (simple hydrogen). • So, about 25 per cent of the matter (by mass) should have turned into helium, while most of the rest was hydrogen. (A little bit of the hydrogen was in the form of deuterium and tritium, but the latter is unstable and quickly decays.)

45. 19.4c Primordial Nucleosynthesis • The fact that helium is observed to be about 25 per cent by mass throughout the Universe (as far as we can tell) strongly supports the big-bang theory: The Universe had to be hot and dense at early times. • The rather uniform relative proportion (abundance) of helium suggests a primordial origin, before stars were born. • Heavier elements, on the other hand, exhibit rather large variations in their relative proportions: In some stars they are rare, and in others they are common, implying an origin that is not primordial or uniform throughout the Universe.

46. 19.4c Primordial Nucleosynthesis • The verification of the predictions of primordial nucleosynthesis is one of the four major pillars on which the big-bang theory rests. (The other three are the observed expansion of the Universe, the existence of the cosmic background radiation, and the observed evolution of the Universe from its beginning about 14 billion years ago.) • Being inconsistent with the competing steady-state theory, it is more compelling than the mere expansion of the Universe. • George Gamow and his students had the right idea when they concluded that the Universe must have been hot at an early stage. • Their mistake was in thinking that all of the elements (instead of just the lightest ones) were synthesized shortly after the big bang. • They didn’t know about nucleosynthesis in the cores of stars.

47. 19.4c Primordial Nucleosynthesis • The approximate observed abundances of the light elements show that the general ideas of primordial nucleosynthesis are correct. • Detailed measurements provide some additional constraints; the exact current proportions reflect conditions in the early Universe. • For example, deuterium is rapidly destroyed, forming helium. • Thus, if the density remained high for a long time (the expansion of the Universe was relatively slow), the current deuterium abundance would be very low. • The fact that deuterium is reasonably abundant right now (about 10-5 of the amount of ordinary hydrogen) tells us that M, the ratio of the average matter density to the critical density, is only about 0.05 (5 per cent) (see figure).

48. 19.4c Primordial Nucleosynthesis • In spite of years of trying, nobody succeeded in observing the fundamental spin-flip line of deuterium, the analogue to the 21-cm line of ordinary hydrogen, until the report of MIT’s Alan Rogers and colleagues in 2005. • Observing in Massachusetts, they used a field of broadband antennas, linked electronically, to detect deuterium in the direction of our Galaxy’s anticenter, that is, the direction opposite to that of our Galaxy’s center. • The background noise in the anticenter direction is so much lower than that toward the center that the deuterium was detectable, in spite of its being weaker. • They found that deuterium nuclei are fewer than hydrogen nuclei by a ratio of 2.3 × 10-5.

49. 19.4c Primordial Nucleosynthesis • The 5 per cent limit measurement of M only refers to normal matter (protons, neutrons, electrons). • There might be additional contributions to M, but they must be “exotic” matter such as neutrinos and “weakly interacting massive particles” (WIMPs). • If we believe that M  0.3, as suggested by studies of clusters of galaxies and the peculiar motions of galaxies (Chapter 18), then a large fraction of the matter in the Universe is not only dark, but must also be composed of some kind of exotic matter! • This conclusion is consistent with the findings of WMAP discussed above (Sec. 19.3c).

50. 19.5 The Inflationary Universe • The original, “standard” hot-big-bang theory is remarkably successful. • Its four main observational foundations are the existence of the cosmic microwave background radiation, the relative proportions of various isotopes of the lightest elements (hydrogen, helium, and lithium), the expansion of the Universe, and the evolution of the Universe over a finite amount of time. • However, there are some puzzling aspects of the Universe that the original big-bang theory cannot explain, at least not without imposing “initial conditions” that seem unlikely and unjustified. • Here we will discuss two of the main problems. • We will also present a brilliant (but still speculative) refinement to the big-bang theory that appears to resolve them by affecting just the first blink-of-an-eye of the Universe’s existence.